8. Resultados
8.1. Germinación in vitro de semillas de mortiño
Figure 5.18shows the cross sections of beads slightly etched with Krolls reagent whose phases are presented in Section 5.6.2. In the cross sections, the observed pores are located along the fusion line. Partially melted Ti-6Al-4V particles are seen mostly near the top of the bead cross section down towards the middle section of the sample. A clear fusion line is also observed with the expectation of a good metallurgical bond between the bead and the substrate. The contact angle between the clad and the substrate is observed to be greater than 100o in all cases. An angle of 150±2o was measured for bead cross section in Figure 5.18(a). This indicates that all the beads presented are suitable for multipass overlap cladding which would result in no inter-run porosity with the proper selection of overlap pitch. Moreover, the micrographs confirm that bead- substrate dilution is reduced as laser power increases to complement result presented in Section5.5.3.
Figure5.19shows magnified images of the box section of the bead cross sections of Figure
5.18showing a Ti rich primary phase (dark contrast) and a eutectic like region of needles and matrix in between the dendrites. The dendritic Ti-rich phase is more apparent in Figure5.19(b) than in (d) or (f ). The TiB reinforcements, which are commonly seen as white needles in the micrograph, are randomly oriented. The TiB needles are of varying lengths and a very narrow width. In Figure 5.19(d), the longest TiB needle observed in the micrograph of bead deposited with a laser power of 1600 W is 30 µm, while 40 µm is measured in Figure 5.19(f ) for bead deposited with 1800 W laser power. This may indicate that the length of the in-situ synthesized TiB reinforcement is positively dependent on the laser power employed to process the feedstock.
In Figure 5.19(f ), the TiB precipitates are well distributed in the Ti phase, while eu- tectic TiB reinforcements which are oriented in the same direction is prevalent in the Ti-rich interdendritic region in Figure 5.19(b). This is a good indication of a strong mixing activity which must have taken place when higher laser power is employed for the cladding process. The higher magnification SEM images (secondary electron) of Figure 5.20 show the presence of what could be partially dissolved TiB2 particles and micro pores throughout the bead cross sections irrespective of the laser power used. In the XRD scan, low intensity peaks of TiB2 phase are identified for the bead processed
Figure 5.18: SE-SEM image of the cross section of single beads showing partially melted Ti-6Al-4V particles around the clad periphery and pores close to the fusion line. Process parameters: Traverse speed = 400 mm/min, Powder feed rate = 10
g/min; Laser power = (a) 1400 W (b) 1600 W and (c) 1800W.
at 1400 W. This may suggest that the presence of partially dissolved TiB2 particles de- creases with increasing laser power. However, there remains a remnant of the unreacted particles when processed with a high laser power such as 1800 W, as seen in Figure
5.20(c). In Figure5.20(d), the dark grey TiB2 particle is observed to have a light grey edge with growth of TiB needles around the edge. This indicates that as TiB2 particles are dissolving into the Ti melt, its boron atoms react with Ti to form TiB reinforcements on cooling.
Micro pores with white halo edges, which are either circular or elliptical in cross sec- tion, are observed in the micrographs. In Figure 5.20(d), a near circular micro pore is measured to have a 1 µm diameter size.
Figure 5.19: SE-SEM images of etched samples with the middle section of bead cross sections showing a uniform and random distribution of TiB reinforcement in (a)&(b) higher magnification of box in Figure5.18(a), (c)&(d) higher magnification of box in
Figure 5.20: SE-SEM images of etched samples showing the presence of TiB2particles and pores in all bead cross sections.
Figure 5.21 shows the micrograph of the top region of the TiB2/Ti-6Al-4V composite beads. Partially dissolved Ti-6Al-4V and TiB2 particles are mostly observed in the re- gion. The cross section of the embedded Ti-6Al-4V is characterised by what could be acicular or martensitic α-Ti structure. In Figure 5.21(b), radial growth of TiB needles is observed to cluster around the partially melted Ti-6Al-4V particle. These TiB pre- cipitates grow from the Ti-6Al-4V particle surface. The TiB clusters had resulted from the TiB2 powder attached onto the Ti-6Al-4V particles. The TiB2 particles attached to the Ti-6Al-4V particle have been preserved even after it has been blown through the powder feeding system. Upon laser irradiation, the TiB2 particles dissolved making region around the Ti-6Al-4V particle boron-rich and reacting with Ti melt to form TiB clusters as observed.
Figure5.22 shows the micrographs of the clad/substrate region of the composite beads. A clear clad/substrate interface is observed at the fusion boundary. Large pores with a variety of morphologies in cross section are observed in the micrographs which are
Figure 5.21: SE-SEM images of etched samples with the top region of the composite bead showing partially dissolved Ti-6Al-4V and TiB2 particles with TiB precipitate
clusters.
located close to the clad/substrate interface. In Figure 5.22(a), a hemispherical recess is observed with a near circular edge. This pore is not a through hole and the recess indicates a Ti-6Al-4V pull-out. However, in Figure 5.22(b) and (c), the observed pores are deep which is suggested to be associated with incomplete fill or gas entrapment.
Figure 5.22: SE-SEM images of etched samples with the clad/substrate regions of the composite showing large pores and a clear fusion boundary line at the clad/substrate
interface.
The SEM images (secondary electron) of these lightly etched TiB2/Ti-6Al-4V composite samples show that the majority of the TiB2particles dissolve in the melt. The dissolution of TiB2 results in the formation of TiB precipitates in the form of needles on cooling. Depending on the process parameters employed, a Ti-rich dendritic or a eutectic-like TiB dispersed in Ti matrix structure is formed. The TiB reinforcements are uniformly and randomly distributed in a Ti matrix characterized as α-Ti phase. Increasing laser power is observed to produce TiB reinforcement with a longer length. Though partially
dissolved TiB2 and Ti-6Al-4V particles are evident in all samples, XRD results show that by employing a higher laser power, partially dissolved particles may be reduced. A clear clad/substrate interface, a few large pores at the fusion line and micro pores are observed in the micrographs. The adherence of the TiB2 powder onto the surface of the Ti-6Al-4V particles was sufficient to survive powder feeding. This promotes a uniform distribution of the reinforcing element (TiB2 particles/TiB whiskers) in the melt pool when compared to the microstructure of the preliminary cladding experiment discussed in Section5.4.1.